1. Field of the Invention
The present invention relates to a tube pump for pumping fluids.
2. Description of the Related Art
Ink-jet type recording devices to date have employed ink-jet recording heads for ejecting ink onto recording paper or other media. An ink-jet recording head of this design ejects ink through nozzles onto recording paper or other medium, and thus there is a risk that ink might not be ejected smoothly if the ink in proximity to the nozzles should become viscous, or if air bubbles should become entrained in the nozzles. For this reason, ink-jet type recording devices are equipped with a head cleaning unit in order to avoid such phenomena.
The head cleaning unit has a capping mechanism including a cap to cover the nozzles, and a pump for creating negative pressure inside the cap; it is designed to effect cleaning by suctioning ink in proximity to the nozzles, by means of a pump. Tube pumps, which have relatively simple structure and are easily made compact, are used as pumps for this purpose (see JP2004-34525A (FIG. 3 etc.), for example). As illustrated in FIG. 7 of JP2004-34525A, the tube pump is designed to suction ink by means of compressing a tube with a roller while moving the roller in the clockwise direction, for example. Specifically, the roller moves while compressing the tube, and each portion of the tube over which the roller has passed recovers from the compressed state to its original state. This recovery of each tube portion generates negative pressure within the tube, and the ink is transported smoothly through the tube by means of this negative pressure. In preferred practice, the negative pressure created inside the tube will be as high a level as possible, and for this purpose it is preferable for the tube to be substantially completely compressible.
FIG. 9A shows a conventional tube 75, prior to being compressed, and FIG. 9B shows its compressed state. As shown in FIG. 9A, when the tube 75 of wall thickness t=1 mm is compressed by the roller 780 in the direction of the arrows in the drawing, the tube 75 collapses to thickness equivalent to 2t, as shown in FIG. 9B. However there was the problem that, as shown in FIG. 9B, in this state, the tube 75 does not sufficiently collapse so that some gaps S remain within the tube and an adequate negative pressure cannot be created during subsequent recovery. Thus, in the past, it was necessary to compress the tube 75 to an even further extent beyond the state depicted in FIG. 9B, in order to further reduce the gaps S shown in FIG. 9B. For example, it was necessary to compress the tube 75 of FIG. 9B by an additional extent of approximately 0.6 mm. Since excessive compression of the tube 75 in this way creates strong reaction force, a correspondingly high level of torque is required of the tube pump motor, which created the problem of lower efficiency of the tube pump.